1. Field of the Invention
The present invention generally relates to a method and apparatus used in metal melting, refining and processing, for example, steel making in an electric arc furnace (EAF), and more particularly, to a method and apparatus for the melting and decarburization of an iron carbon melt.
2. Description of Background Art
An electric arc furnaces (EAFs) make steel by using an electric arc to melt one or more charges of scrap metal which is placed within the furnace. Modem EAFs also make steel by melting DRI (direct reduced iron) combined with the hot metal from a blast furnace. In addition to the electrical energy of the arc, chemical energy is provided by auxiliary burners using fuel and an oxidizing gas to produce combustion products with a high heat content to assist the arc.
If the EAF is used a scrap melter, the scrap burden is charged by dumping it into the furnace through the roof opening from buckets which also may include charged carbon and slag forming materials. A similar charging method using a ladle for the hot metal from a blast furnace may be used, along with injection of the DRI by a lance, may be used to produce the burden.
In the melting phase, the electric arc and burners melt the burden into a molten pool of metal, called an iron carbon melt, which accumulates at the bottom or hearth of the furnace. Typically, after a flat bath has been formed by melting of all the burden introduced, the electric arc furnace enters a refining and/or decarburization phase. In this phase, the metal continues to be heated by the arc until the slag forming materials combine with impurities in the iron carbon melt and rise to the surface as slag. When the iron carbon melt reaches a boiling temperature, the charged carbon in the melt combines with any oxygen present in the bath to form carbon monoxide bubbles which rise to the surface of the bath. Generally, at this time high velocity, usually supersonic, flows of oxygen are blown into the bath with either lances or burner/lances to produce a decarburization of the bath by the oxidation of the carbon contained in the bath.
By boiling the bath with the injected oxygen, the carbon content of the bath can be reduced to a selected level. If an iron carbon melt is under 2% carbon it becomes steel. EAF steel making processes typically begin with burdens having less than 1% carbon. The carbon in the steel bath is continually reduced until it reaches the content desired for producing a specific grade of steel, down to less than 0.1% for low carbon steels.
With the imperative to decrease steel production times in electric arc furnaces, it becomes necessary to deliver effective decarburizing oxygen to the iron carbon melt as early in the steel making process as possible. Conventional burners mounted on the water cooled side walls of the furnace generally must wait until the melting phase of the process is substantially complete before starting high velocity injection of oxygen for the decarburization process. These burners can not deliver effective high velocity oxygen before then because unmelted scrap is in the way of the injection path and would deflect the oxygen flow. Additionally, the bottom of the electric arc furnace is spherical shaped and the melted scrap forms the melt in the middle of the furnace first and then rises filling up the sides. Early in the melting phase a high velocity oxygen stream has no effective way to reach the iron carbon melt surface to penetrate it and decarburize the melt.
Therefore, it would be highly advantageous to reduce the melting phase of an electric arc furnace so that high velocity oxygen could be injected sooner and decarburize the melt faster.
One way to shorten the melting phase is to add substantially more energy with the burners at early times in the melting phase to melt the scrap faster. There are, however, practical considerations with conventional side wall mounted burners that limit the amount of energy which can be introduced into the furnace and the rate at which it can be used efficiently. When scrap is initially loaded into the furnace, it is very near the flame face of the burner and the danger of a flash back of the flame against the side wall where it is mounted is significant. The panels where the burners are mounted are typically water cooled and a burn through of a water carrying element in an electric arc furnace is a safety concern. To alleviate this concern, many fixed burners are run at less than rated capacity until the scrap is melted some distance away from the face of the burner. Only after the burner face has been cleared does the burner operate to deliver its maximum energy. Another problem to increasing the energy added during the early part of the melting phase is that the flame of the burner is initially directed to a small localized area of the scrap on the outside of the scrap burden. It is difficult to transfer large amounts of energy of the burner from this localized impingement to the rest of the scrap efficiently. Until the burner has melted the scrap away from its face and has opened a larger heat transfer area, increasing a burner to maximum output would only result in a substantial portion of the energy in the combustion gases heating the atmosphere.
Therefore, it would be advantageous to be able to increase the amount of energy applied by a burner during the early part of the melting phase which did not produce a risk of flash back for the water cooled panels of the upper shell of the furnace.
It would also be advantageous to use this increased amount of energy more efficiently and to transfer increased portions of the energy to the scrap burden.
Conventionally, oxygen is blown or injected into the iron carbon melt where it reacts with the carbon in the molten bath to lower the carbon content to the level desired for the end product. In general, the rate of decarburization in an electric arc furnace is determined by the carbon concentration of the iron carbon melt, the oxygen injection rate and the surface area of the reactions sites. At higher bath carbon concentrations, the reaction rate is not significantly limited by the availability of carbon to enter the reaction. However, as the bath carbon decreases to concentrations under approximately 0.15%-0.20% of carbon, it becomes increasingly difficult to achieve an acceptable rate. This is because the carbon concentration of the bath becomes the decarburization rate determining factor. The decarburization rate, after the critical carbon content has been reached, is dominated by mass transfer of the carbon and the carbon concentration.
The prior art practice to decarburize an iron carbon melt is characterized by the localized application of a large volume of oxygen by means of devices such as lances and burner/lances. Due to the localized nature of this process, the decarburization rate depends on the rate of oxygen injection to the bath, the carbon concentration and the mass transfer of carbon to the reaction area. At lower carbon levels, the iron oxide concentration in the slag reaches levels greater than equilibrium would allow, due to depleted local carbon concentration and poor mass transport. This causes greater refractory erosion, loss of iron yield, increased requirements for alloys, and a low efficiency of oxygen utilization.
Therefore, it would be advantageous to provide a method and apparatus to supply oxygen for efficient decarburization of the iron carbon melt at all carbon concentrations. A method that increased the number of reaction zones and supplied significantly more effective oxygen early in the process would be advantageous because it would shorten the duration of decarburization. Particularly important is the efficiency of the oxygen supply after the iron carbon melt reaches a low carbon content in order to maximize the decarburization rate, without over oxidizing the slag and producing excess amounts of FeO. This would reduce operating costs by improving oxygen efficiency, reducing excess iron oxidation, improving alloy recovery, and increasing productivity.
The conventional oxygen injection equipment that has been used for decarburization is not generally suited for efficient introduction of oxygen into an iron carbon melt. The use of retractable consumable or water cooled lances through the slag door opening, or through the side wall, is always limited by the space available to position the equipment around the furnace. Its location is usually only practical in the quadrant of the furnace shell near the slag door. The basic furnace design, required manipulator movement, the size of the manipulator and the necessity of operators to observe the use as well as to allow easy access dictates the location of the manipulator. The design is also responsible for the introduction of a substantial amount of cold ambient air into the process through the slag door or side wall opening during manipulation of the moveable lance. These large amounts of cold air reduce the efficiency of the process and also contribute to a nitrous oxide increase in the furnace atmosphere. There is also a significant delay in moving the lance into the furnace through the scrap burden. The scrap must be melted in front of the lance before it can advanced into the hot reaction zone of the furnace where it can deliver effective oxygen.
Fixed oxygen injection equipment such as a burner/lance mounted on the side wall water cooled panels, or upper shells of the furnace are positioned a significant distance away from the iron carbon melt. That distance is generally determined by the geometry of the furnace side wall with respect to the transition from the upper shell to the lower shell of the furnace which forms a step. The water cooled part of the upper shell where the burner/lances have been located is mounted on the lower shell or refractory, but typically about 15-24xe2x80x3 back from the hot face of the refractory. Because a fixed burner/lance has had to fire over this step, the traditional fixed wall oxygen injection equipment had to be located about 45xe2x80x3 above the molten bath in an attempt to deliver oxygen with the optimum angle of impingement. This distance and the angle requires the length of the injected stream of oxygen to be about 65xe2x80x3 or longer.
It is very difficult to deliver 100% of an oxygen stream effectively to a reaction zone at these distances. The amount of effective delivery of a high velocity (high kinetic energy) oxygen stream to the iron carbon melt is proportional to the diameter of the oxygen injector opening (in the case of a converging-diverging nozzle the bore size of the nozzle) and the length the oxygen jet travels to the iron carbon melt. Thus, increasing the bore size increases the total amount of effective oxygen reaching the iron carbon melt, but may also result in an increase of unused oxygen in the furnace atmosphere. Another method of enhancing the effectiveness of an oxygen stream for decarburization has been to shroud it with the products of combustion, or other gases. The shrouding tends to maintains the stream together over a longer distance thereby increasing its penetrating power. In spite of the effectiveness gained by shrouding, a significant amount of the oxygen is lost to the furnace environment causing several detrimental effects on operations. Initially, there is the increased cost of the shrouding gases and specialized equipment to form the shroud. The excess oxygen causes damage to the side wall panels, erosion of the shell refractory, development of excessive iron oxide in the slag, excessive electrode oxidation, reduction in the delta life, and may cause over heating of the furnace evacuation system.
Moreover, conventional oxygen injection equipment that has been used for decarburization is not generally suited to varying the oxygen supply rate over substantial ranges. Fixed oxygen injection equipment such as burner/lances mounted on the side wall panels of the furnace have the problem that they are positioned some distance away from the surface of the iron carbon melt. These fixed lances obtain their oxygen injection capability by a supersonic or high velocity nozzle which accelerates the oxygen such that its kinetic energy is enough to penetrate the surface of the iron carbon melt even from considerable distances. If the flow rates of these injectors are reduced significantly, the high velocity nozzles will not impart enough gas velocity to the oxygen to penetrate and create an efficient reaction zone for decarburization.
The invention provides a method and apparatus for improving the melting and decarburizing phases of an iron carbon melt. More particularly, the method and apparatus are useful for the steel making operation of an electric arc furnace.
According to one embodiment of the invention for steel making, the duration of the melting phase is decreased by adding increased amounts of energy early in the melting phase with the combustion products of a burner/lance flame which is directed into a more efficient combustion reaction zone, preferably below the refractory line of the furnace. When the burner flame is generated at this position of the furnace, several distinct advantages pertain to the steel making process. Melting a path for an injection of high velocity oxygen is facilitated because there is less path length to clear and it can be done faster. The time for melting the path length is further reduced by increasing the burner output to its maximum rating early in the melting phase. With a positioning of the flame below the refractory line, there is substantially less possibility for a flash back and the refractory can withstand such operation without catastrophic failure. The process of melting a clear path is also faster because the flame works in a hotter area closer to the electric arc. Further, the hot combustion gases flow upward through the total burden of scrap and cause additional energy transfer instead of heating the furnace atmosphere.
In addition to the efficiency gain caused by starting the oxidizing gas early in the melting cycle, a method for decarburization includes a process for increasing the efficiency of the oxidizing gas utilization in the iron carbon melt. More particularly, the method includes supplying a plurality of reaction zones with an oxidizing gas to decarburize an iron carbon melt with an efficient oxygen supply profile which is related to the carbon content of the melt. The multiple reaction zones are used to increase the amount of oxygen which can be effectively used for decarburization of the melt by increasing the reaction zone area and by making each reaction zone more efficient. Each reaction zone is more efficient because the surface dynamics of the process are occurring in multiple localized areas. The carbon being depleted in each local area is replenished more quickly than a single large area because of the better mass transport. This will lower the duration the decarburization process and at the same time oxidize less iron.
Preferably, after the critical carbon content has been reached, or optionally a carbon content near to the critical content below 0.2%, the multiple reaction zones can be supplied with reduced amounts of oxygen which are dependent upon the amount of carbon content at the particular time of the process. Preferably, the total oxygen supply profile for the multiple zones can approximate an exponential decay, similar to the demand for oxygen by the decreasing carbon content. Using multiple reaction zones during this phase of the decarburization process several distinct advantages. Because this phase of the decarburization process is dependent upon the surface reaction kinetics and carbon content, as the carbon content decreases, the multiple localized areas become even more efficient compared to a single reaction area. The increase in efficiency is greater because of the increased total reaction area and decreased time for the mass transport of the carbon in each zone. Further, multiple reaction zones combined with the shorter distance for the oxygen to travel to the molten metal in each zone creates several areas of deep penetration of the melt to increase agitation which is beneficial to the reaction.
A preferred embodiment of the invention includes a plurality of injection apparatus which efficiently supply high velocity combustion gases and oxidizing gas to each reaction zone. The injection apparatus preferably comprise fixed burner/lances or lances which are capable of injecting combustion gases and high velocity oxygen, preferably at supersonic velocities. In the illustrated embodiment, the high velocity oxygen is developed by a nozzle structure of a burner/lance which accelerates the oxidizing gas to supersonic velocities. The nozzle structure of the burner/lance also includes fuel and secondary oxidizing gas jets which are used after combustion to form a shroud around the high velocity oxygen and maintain its penetrating power.
The burner/lance or lance is then mounted in a protective enclosure which allows the nozzle structure to be located closer to the surface of the melt and closer to the center of the furnace than other fixed burners mounted on the side wall panels. The protective enclosure in the preferred embodiment is a fluid cooled enclosure having at least one face adapted to withstand the harsh environment of the inside of the furnace. The burner lance is mounted at an optimal attack angle through a mounting aperture in this face.
Mounting the burner lance in a protective enclosure produces several advantages. The protective enclosure moves the burner flames and high velocity oxygen flow away from the wall of the furnace and closer to the edge of the refractory. This greatly reduces or eliminates the chance that the burner flames or the high velocity oxygen flow will reflect (flashback) against the furnace wall and create damage. Advantageously for the high velocity oxidizing gas flow, the flow has a shorter distance to travel to reach the melt compared to a lance mounted on the side wall. The shorter flow path length permits the oxidizing gas flow to impinge on the melt with a higher velocity and more concentrated flow pattern which causes a more efficient and rapid decarburization. The shorter flow path length also eliminates the need for excessive shrouding gasses and oxygen jets with large flow rates. This significantly reduces the negative oxidizing effects to the furnace because of excess oxygen.
Further, the shorter flow path length provided by the enclosure and multiple zones permitting reduced flow rates at each zone, allows the flow of the oxidizing gas at each zone to be controlled over a substantial range while still maintaining high velocity and efficient penetrating power for the melt at each zone. The capability of the preferred apparatus to permit the control of the oxidizing gas flow rate over a substantial range while still maintaining efficient decarburizing velocity facilitates the supply of an oxidizing gas profile to each reaction zone which is related to the carbon content of the melt.
These and other objects, aspects and features of the invention will be more clearly understood and better described when the following detailed description is read in conjunction with the attached drawings, wherein similar elements throughout the views have the same reference numerals, and wherein: